Method of manufacturing silicon epitaxial wafer

ABSTRACT

A silicon epitaxial layer  2  is grown in vapor phase on a silicon single crystal substrate  1  manufactured by the Czochralski method, and doped with boron so as to adjust the resistivity to 0.02 Ω·cm or below, oxygen precipitation nuclei  11  are formed in the silicon single crystal substrate  1 , by carrying out annealing at 450° C. to 750° C., in an oxidizing atmosphere, for a duration of time allowing formation of a silicon oxide film only to as thick as 2 nm or below on the silicon epitaxial layer  2  as a result of the annealing, and thus-formed silicon oxide film  3  is etched as the first cleaning after the low-temperature annealing, using a cleaning solution. By this process, the final residual thickness of the silicon oxide film can be suppressed only to a level equivalent to native oxide film, without relying upon the hydrofluoric acid cleaning.

RELATED APPLICATIONS

This application claims the priorities of Japanese Patent ApplicationNo. 2004-245691 filed on Aug. 25, 2004, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of manufacturing a silicon epitaxialwafer having a silicon epitaxial layer formed by vapor phase growth on asilicon single crystal substrate doped with a relatively highconcentration of boron.

2. Description of the Related Art

Silicon epitaxial wafers, having a silicon epitaxial layer grown invapor phase on a silicon single crystal substrate manufactured by theCzochralski method (simply referred to as the CZ method, hereinafter),while being doped with a high concentration of boron so as to adjust theresistivity to as low as 0.02 Ω·cm or below (referred to as p⁺CZsubstrate, hereinafter), have widely been used, for example, for thepurpose of preventing latch-up, and of eliminating any defects from adevice forming region.

The p⁺CZ substrate has a large number of oxygen precipitation nucleiformed in a crystal pulling process over a period from solidification ofthe crystal to cooling down to room temperature. Size of the oxygenprecipitation nuclei is generally as very small as 1 nm or below. Theoxygen precipitation nuclei can grow up to oxygen precipitate, if thenuclei are kept at a temperature not lower than a nuclei formingtemperature and not higher than a certain critical temperature relevantto re-solid solubility into the bulk of silicon single crystal. Theoxygen precipitate is a sort of BMD (bulk micro-defect) causative ofdeterioration of withstand voltage and current leakage, so that it isdesired to be formed as less as possible in the device forming region.The oxygen precipitate can, however, effectively be used as a getter forheavy metal components in a device manufacturing process, in regions ofthe substrate not destined for device formation, so that it has been ageneral practice to intentionally form the oxygen precipitate in thesilicon single crystal used for growth in the manufacture of siliconepitaxial wafers, only to a degree not causative of nonconformities suchas warping. The effect of gettering heavy metals by the oxygenprecipitate is one of so-called IG (intrinsic gettering) effects.

By the way, the oxygen precipitation nuclei are known tore-solid-solubilize themselves into the bulk of silicon single crystaland disappear, if the substrate is kept at a temperature higher than theabove-described critical temperature. As for the silicon epitaxialwafers, vapor phase growth process of the silicon epitaxial layercorresponds to high temperature annealing at 1,100° C. or above wherethe nuclei disappear, and this means that the oxygen precipitationnuclei, resided abundantly before the vapor phase growth, can largely bereduced due to thermal history during the vapor phase growth. Reductionin the oxygen precipitation nuclei suppresses formation of the oxygenprecipitate in the process of manufacturing semiconductor devices, evenif the initial oxygen concentration in the adopted silicon singlecrystal substrate is high, and achieving the IG effect is less hopeful.

Aiming at solving this problem, there is proposed a method of subjectinga silicon epitaxial wafer to low temperature annealing at 450° C. to750° C., both ends inclusive, so as to newly produce the oxygenprecipitation nuclei in the p⁺CZ substrate, and then tomiddle-temperature annealing (a temperature range between those forlow-temperature annealing and high-temperature annealing) to therebygrow the oxygen precipitate (Japanese Laid-Open Patent Publication Nos.H9-283529, H10-270455, and International Patent Disclosure WO01/056071).

Japanese Laid-Open Patent Publication No. H9-283529 describes, in thesection titled “Preferred Embodiments of the Invention”, thatlow-temperature annealing aimed at forming the oxygen precipitationnuclei in an oxygen atmosphere results in formation of silicon oxidefilm on the surface of the silicon epitaxial wafer. Thus-formedunnecessary silicon oxide film can be removed by hydrofluoric acidcleaning, as is well known in the art. Removal of the silicon oxide filmby the hydrofluoric acid cleaning, however, raises the particle level onthe surface of the silicon epitaxial wafer after the cleaning. Adoptionof the hydrofluoric acid cleaning aimed at removing the silicon oxidefilm after the low-temperature annealing also increases the number ofprocess steps, and consequently increases costs for manufacturing thesilicon epitaxial wafers.

It is therefore a subject of this invention to provide a method ofmanufacturing a silicon epitaxial wafer, on the premise that annealingfor forming oxygen precipitation nuclei is carried out in an oxidizingatmosphere, capable of suppressing the final residual thickness of thesilicon oxide film formed during the annealing to a level equivalent tothat of native oxide film without adopting hydrofluoric acid cleaning,and furthermore suppressing increase in the particles after thecleaning.

SUMMARY OF THE INVENTION

Aimed at solving the above-described subjects, a method of manufacturinga silicon epitaxial wafer of this invention includes:

a vapor phase growth step allowing a silicon epitaxial layer to grow invapor phase on a silicon single crystal substrate manufactured by theCzochralski method, and doped with boron so as to adjust the resistivityto 0.02 Ω·cm or below;

a low-temperature annealing step forming, following the vapor phasegrowth step, oxygen precipitation nuclei in the silicon single crystalsubstrate, by carrying out annealing at 450° C. to 750° C., both endsinclusive, in an oxidizing atmosphere, for a duration of time allowingformation of a silicon oxide film only to as thick as 2 nm or below onthe silicon epitaxial layer as a result of the annealing; and

a cleaning step, as the first cleaning step after the low-temperatureannealing step, etching the silicon oxide film formed in thelow-temperature annealing step, using a cleaning solution composed of amixed solution of ammonia, hydrogen peroxide and water,

all of these steps being executed in this order.

As cleaning solutions for cleaning the silicon epitaxial wafers havingwidely been accepted in general, there are three known cleaningsolutions proposed by RCA, a US company:

(1) a mixed solution of ammonia, hydrogen peroxide and water(representative composition is SC-1 cleaning solution described later),used for removing organic pollution and particles;

(2) an aqueous hydrofluoric acid solution, for removal of silicon oxidefilm; and

(3) a mixed solution of hydrochloric acid, hydrogen peroxide and water(representative composition is SC-2 solution described later), used forremoving surficial metal impurities.

All of these solutions are almost standardized in this industry, andpurposes of use of the individual solutions are clearly discriminated asshown in the above, so that it could be said that there is almost noopportunity of using them beyond their purposes of use. Considering thesubjects of this invention, it is therefore sensible to use (2) ahydrofluoric acid solution, for the purpose of removing the siliconoxide film formed in the low-temperature annealing step. The cleaningusing hydrofluoric acid (also referred to as hydrofluoric acid cleaning,hereinafter), however, increases the particle count on the wafer surfaceas described in the above. For the purpose of removing the particlesundesirably increased in the process of the hydrofluoric acid cleaning,it is therefore a commonsense for those skilled in the art to furthercarry out the SC-1 cleaning using the mixed solution (1) of ammonia,hydrogen peroxide and water, that is, so-called, two-step cleaning.

This invention was completed paying by attention to a certain level ofetching effect of the cleaning solution (1) composed of the mixedsolution of ammonia, hydrogen peroxide and water over the silicon oxidefilm, although of course only to a level smaller than that attainable bythe hydrofluoric acid cleaning solution (2), so as to turn over theabove-described commonsense such that “hydrofluoric acid isindispensable for removal of the silicon oxide film”. In other words,conditions for the low-temperature annealing (temperature and time) forforming the oxygen precipitation nuclei are selected, so as to make theamount of increase in the silicon oxide film in the process oflow-temperature annealing under an oxidizing atmosphere almostcomparative to the thickness removable by etching using the cleaningsolution described above. More specifically, temperature of thelow-temperature annealing is set within the range from 450° C. or aboveand 750° C. or below, in which formation of the oxygen precipitationnuclei in the p⁺CZ substrate becomes distinct, and the annealing time isset so as to make the thickness of the oxide film formed on the siliconepitaxial layer as a result of the annealing as small as 2 nm or below.Following the low-temperature annealing step under these conditions, thesilicon oxide film is etched, using the cleaning solution (1), as thefirst cleaning step after the low-temperature annealing step.

In the above-described method, the thickness of the silicon oxide filmincreases in the low-temperature annealing, but the conditions of theannealing are selected so as to make the thickness after thelow-temperature annealing to as small as 2 nm or below. Thereafter, thewafer is directly subjected to cleaning using the cleaning solution (1),which has generally been understood as being used for removal ofparticles, so as to make full use of a latent and limitative etchingability owned by the cleaning solution, so that the increased portion ofthe silicon oxide film as a result of the low-temperature annealing caneffectively be reduced. As a consequence, while being on the premise ofannealing the wafer in order to form the oxygen precipitation nuclei inan oxidizing atmosphere, the final residual thickness of the siliconoxide film formed during the annealing can be suppressed only to a levelequivalent to native oxide film, without relying upon the hydrofluoricacid cleaning. The cleaning solution (1) is intrinsically excellent inthe effect of removing particles, so that also increase in the particlesafter the cleaning can be suppressed. In this case, it is feasibleenough to reduce the particle count on the silicon epitaxial layer afterthe cleaning step, to a level lower than the particle count attainedbefore the cleaning step, and thereby nonconformities ascribable to theparticles can effectively be reduced.

The thickness of the silicon oxide film after the cleaning stepnaturally becomes smaller than that attained after the low-temperatureannealing, contributed by the etching effect of the above-describedcleaning solution. On the other hand, from the viewpoint of surfaceprotection of the silicon epitaxial layer, it is preferable that thesilicon oxide film remains to as thick as 1 nm or above, equivalent tothe level of native oxide film, even after the cleaning step. Thecleaning solution (1) has a cleaning activity ascribable to ammonia,enhanced through addition of hydrogen peroxide which is a relativelystrong oxidizer, so that the solution can repair the silicon oxide filmto as thick as 1 nm or more, by virtue of the oxidizing ability ofhydrogen peroxide, even when the silicon oxide film is excessively thin.

Next, the resistivity of the silicon single crystal substrate exceeding0.02 Ω·cm fails in ensuring a density of formation of the oxygenprecipitation nuclei sufficient for ensuring the IG effect, becauseconcentration of boron capable of promoting the oxygen precipitationbecomes too small, and consequently the number of oxygen precipitationnuclei decreases. In particular in this invention, the low-temperatureannealing is finished within a relatively short period of time, so as tocontrol the thickness of the silicon oxide film to as small as 2 nm orbelow as described in the above, so that it is important to set theresistivity of the silicon single crystal substrate to as small as 0.02Ω·cm or below, in view of ensuring a necessary number of oxygenprecipitation nuclei. From this point of view, the resistivity of thesubstrate is preferably set to less than 0.014 Ω·cm. On the other hand,in view of suppressing warping of the substrate or the like due toexcessive increase in the density of formation of the oxygenprecipitate, the resistivity of the substrate is preferably set to 0.011Ω·cm or above.

The temperature of the low-temperature annealing of lower than 450° C.extremely reduces the number of formation of the oxygen precipitationnuclei, and on the contrary, the temperature exceeding 750° C. resultsin insufficient number of formation of the oxygen precipitation nuclei,due to excessively low degree of super-saturation of the interstitialoxygen. The temperature of the low-temperature annealing is, therefore,set in the range from 450° C. to 750° C., both ends inclusive.

The initial oxygen concentration in the silicon single crystal substrateis preferably 6×10¹⁷ cm⁻³ to 10×10¹⁷ cm⁻³, both ends inclusive. Theinitial oxygen concentration of less than 6×10¹⁷ cm⁻³ fails in ensuringa sufficient density of formation of oxygen precipitate, andconsequently fails in fully expecting the IG effect. On the contrary,the initial oxygen concentration exceeding 10×10¹⁷ cm⁻³ results inexcessive density of formation of oxygen precipitate, and may be morelikely to induce sharp increase in wafer deformation such as warping. Itis to be noted that the oxygen concentration in this patentspecification is expressed in unit based on criteria of JEIDA (acronymfor Japanese Electronic Industry Development Association, renamed JEITA(Japan Electronics and Information Technology Industries Association) atpresent).

By the low-temperature annealing carried out within the above-describedtemperature range after the vapor phase growth step, the oxygenprecipitation nuclei once disappeared or decreased during the vaporphase growth step can be recovered up to a density of formationnecessary for ensuring the IG effect. By further carrying out themiddle-temperature annealing at a temperature higher than that in thelow-temperature annealing and lower than that in the vapor phase growth,more specifically 800° C. or above and lower than 1,100° C., the oxygenprecipitation nuclei can be grown up to oxygen precipitate.

A cleaning solution adoptable to this invention is the SC-1 (standardcleaning 1) solution. In general, a SC-1 cleaning solution having aratio of mixing by volume of (28 wt % aqueous ammonia solution):(30 to35 wt % hydrogen peroxide):(water) of 1:(1 to 2):(5 to 7), both endsinclusive, can decrease the thickness of the silicon oxide film to alevel of native oxide film without problems, and show a large effect ofremoving particles, so far as the silicon oxide film is limited to asthick as 2 nm or below before the SC-1 cleaning. In recent years,various improvements have been made on the composition of the SC-1cleaning solution, wherein, for example, Morita et al. OYO BUTSURI, Vol.59, No. 11, pp. 79-80, 1990) introduces a composition of a cleaningsolution of (aqueous NH₃):(aqueous H₂O₂):H₂O=1:1:(10 or 15) reduced inthe chemical contents as compared with conventional ones, and claimsthat cleaning property of the SC-1 cleaning depends on ratio ofconcentration of ammonia and hydrogen peroxide, wherein ratio of wateris irrespective of etch rate under a constant ratio of ammonia andhydrogen peroxide. Also in the invention described in Japanese Laid-OpenPatent Publication No. H4-107922, volumetric ratio of hydrogen peroxidein a cleaning solution is made larger than that of an aqueous ammoniumsolution, and ratio of water is increased, for the purpose of loweringthe amount of etching and costs for the chemicals. The inventiondescribed in Japanese Laid-Open Patent Publication No. H7-142435decreases amount of use of ammonia by controlling ammonia concentrationin the SC-1 cleaning solution lowered from 4.3 wt % which is thegenerally adopted concentration, down to a limited range of 2.0 to 3.5wt %.

Depth of etching in the silicon oxide film attained by a singleexecution using the SC-1 cleaning solution is approximately 1 nm, sothat the silicon oxide film having a thickness of 2 nm or smaller beforethe cleaning step may be reduced by etching in the thickness thereofonly to as thin as less than 1 nm, but the final thickness of thesilicon oxide film is stabilized at approximately 1 nm, as a result ofthe above-described repairing effect by hydrogen peroxide. This isadvantageous also in terms of improving uniformity in thickness of thesilicon oxide film after the cleaning. It is to be noted herein that thethickness denoted as X nm in this patent specification means that thethickness falls in the range of ±10% centered round X nm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory drawing sequentially showing process steps inan exemplary method of manufacturing a silicon epitaxial wafer accordingto this invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Paragraphs below will describe best modes for carrying out thisinvention, referring to the attached drawings.

FIG. 1 is an explanatory drawings sequentially showing process steps ofan exemplary method of manufacturing a silicon epitaxial wafer accordingto this invention. First, a p⁺CZ-type silicon single crystal substrate 1(simply referred to as “substrate 1”, hereinafter) adjusted as having aresistivity of 0.02 Ω·cm or below through addition of boron, and aninitial oxygen concentration of 6×10¹⁷ cm⁻³ to 10×10¹⁷ cm⁻³, both endsinclusive, is prepared (FIG. 1, step A). The lower limit of theresistivity of the substrate 1 is specified by the solid solubilitylimit of boron into silicon, wherein resistivity of the substrate 1currently available for manufacturing of semiconductor devices is 0.0005Ω·cm (0.5 mΩ·cm) or above. The substrate 1 has oxygen precipitationnuclei 11 formed therein over the period from solidification of thesilicon single crystal in the crystal pulling process to cooling down toroom temperature.

Next in a vapor phase growth step, a silicon epitaxial layer 2 is grownin vapor phase on the substrate 1 at 1,100° C. or above, to obtain asilicon epitaxial wafer 50 (FIG. 1, step B). Because the vapor phasegrowth process is carried out at a high temperature of 1,100° C. orabove, most of the oxygen precipitation nuclei 11 in the substrate 1formed during the crystal pulling process are solid-solubilized.

After completion of the vapor phase growth step, the silicon epitaxialwafer 50 is loaded in an annealing furnace not shown, and subjected tolow-temperature annealing at 450° C. to 750° C., both ends inclusive, inan oxidizing atmosphere, so as to reproduce the oxygen precipitationnuclei 11 in the substrate 1, to thereby obtain a silicon epitaxialwafer 60 (FIG. 1, step C). The oxidizing atmosphere is typically anatmosphere obtained by diluting a dry oxygen with an inert gas such asnitrogen, or may be a pure dry oxygen. The low-temperature annealing ata temperature below 450° C. extremely retards diffusion of theinterstitial oxygen, and makes the oxygen precipitation nuclei 11 lesslikely to be produced. On the other hand, the low-temperature annealingexceeding 750° C. lowers the degree of super-saturation of theinterstitial oxygen, and again makes the oxygen precipitation nuclei 11less likely to be produced.

In this invention, the low-temperature annealing described above iscarried out only for a duration of time allowing the silicon oxide film3 to grow up to a thickness t1 of 2 nm or less as a result of theannealing. The annealing is preferably carried out within a short periodof time of 4 hours or less, although a specific annealing time may varydepending on the annealing temperature. The annealing time less than onehour may, however, result in only an insufficient formation of theoxygen precipitation nuclei in the substrate 1, so that the annealingtime is set to one hour or longer. Because the annealing in this case isonly effected for a short duration of time, the substrate having aresistivity of higher than 0.02 Ω·cm limits the density of the oxygenprecipitate formable during the succeeding middle-temperature annealingonly to as large as the level of 10⁸ cm⁻³ and consequently makes itdifficult to form the oxygen precipitate to the level of 10⁹ cm⁻³ orabove for which a sufficient level of IG (intrinsic gettering) effectcan be expected. It is therefore necessary to use the substrate 1 havinga resistivity of 0.02 Ω·cm or below.

After completion of the low-temperature annealing step described above,cleaning using the SC-1 cleaning solution (also referred to as “SC-1cleaning”, hereinafter) is carried out as the first cleaning, so as toremove any particles adhered on the surface of the silicon epitaxialwafer 60, and so as to etch the silicon oxide film 3 formed on thesurface of the wafer 60 (FIG. 1, step D). The SC-1 cleaning is carriedout generally in the cleaning process after vapor phase growth, andmoreover, it is a general practice to carry out the hydrofluoric acidcleaning prior to the SC-1 cleaning when it is desired to remove thesilicon oxide film. The hydrofluoric acid cleaning, however, tends toraise the particle level on the wafer surface, and increases the numberof process steps, so that this invention makes use of an etching effectowned by the SC-1 cleaning solution to thereby reduce the thickness t1of the silicon oxide film 3, without relying upon the hydrofluoric acidcleaning.

In the SC-1 cleaning, approximately 1 nm of silicon oxide film 3 can beetched by a single execution of the cleaning step, by virtue of theetching action of ammonia contained in the cleaning solution. Becausethe thickness t1 of the silicon oxide film 3 formed in thelow-temperature annealing step in this invention is 2 nm or less,thickness t2 of the silicon oxide film 3 after the cleaning step issupposed to be thinner than 1 nm. However, for the case where thethickness t2 of the silicon oxide film 3 becomes thinner than 1 nm, theoxidizing action of hydrogen peroxide becomes distinct, and can thickenthe silicon oxide film up to 1 nm or more. However, for the case wherethe thickness t1 of the silicon oxide film 3 formed in thelow-temperature annealing step is thicker than 2 nm, it is difficult toreduce the thickness to as thin as approximately 1 nm by a singleexecution of the SC-1 cleaning step. It is to be noted that, in thecleaning step described above, the SC-1 cleaning may further be followedby the SC-2 cleaning (using a mixed solution of hydrochloric acid,hydrogen peroxide and water), if necessary.

After completion of the cleaning step, the resultant silicon epitaxialwafer 100 is subjected to the middle-temperature annealing formanufacturing ICs or the like, for example, in the device formingprocess (FIG. 1, step E), wherein the oxygen precipitation nuclei 11grow by the middle-temperature annealing, and thereby the oxygenprecipitate 12 having the gettering effect are formed. The annealingcarried out in an oxidizing atmosphere can thicken the silicon oxidefilm 3 on a silicon epitaxial wafer 200.

It was experimentally confirmed how the amount of change in the particlelevel before and after the cleaning differs between the cases where thehydrofluoric acid cleaning was adopted or not before the SC-1 cleaning.Results will be explained below. When the silicon eptaxial wafer havingthe silicon oxide film formed thereon was subjected to the hydrofluoricacid cleaning, followed by the SC-1 and the SC-2 cleaning, particleshaving a diameter of 0.1 μm or larger increase by an average of 0.5particles/wafer by a series of these cleaning steps. In contrast, forthe case where the SC-1 cleaning was not preceded by the hydrofluoricacid cleaning, particles decrease by an average of 0.8 particles/wafer.

That is, when the silicon epitaxial wafer 50, obtained by allowing thesilicon epitaxial layer 2 to grow in vapor phase on the p⁺CZ-typesilicon single crystal substrate 1 having a resistivity of 0.02 Ω·cm orbelow, is subjected to the low-temperature annealing in an oxidizingatmosphere for the purpose of forming the oxygen precipitation nuclei11, control of the thickness t1 of the silicon oxide film 3 formedthereon to as small as 2 nm or below makes it possible to adjust thethickness t2 of the silicon oxide film 3 to approximately 1 nm,equivalent to the level of native oxide film, by the SC-1 cleaning. Evenfor the case where the SC-1 cleaning directly follows the vapor phasegrowth without being preceded by the low-temperature annealing, thesilicon oxide film can be adjusted to as thin as approximately 1 nm, sothat the thickness t2 of the silicon oxide film can be controlled to alevel equivalent to that of native oxide film, without relying upon thehydrofluoric acid cleaning which tends to increase the particle level.

EXAMPLES

Paragraphs below will further specifically explain this invention,referring to Example. It is to be noted that oxygen concentration in thesilicon single crystal substrate described in these Examples is obtainedby converting a value measured by the inert gas fusion method, usingcorrelation between results obtained for a substrate having a standardresistivity (1 to 20 Ω·cm) by the Fourier transformation infraredspectro-photometry and the inert gas fusion method.

The boron-doped silicon single crystal substrate 1 having a resistivityof 0.011 Ω·cm and an initial oxygen concentration of 6.7×10¹⁷ cm⁻³ (13.4ppma) was prepared, and the silicon epitaxial layer 2 having aresistivity of 20 Ω·cm and a thickness of 5 μm was grown in vapor phaseat 1,100° C., on the (100) main surface of the substrate 1, to therebyobtain the silicon epitaxial wafer 50. Next, thus-obtained siliconepitaxial wafer 50 was subjected to the low-temperature annealing at650° C. for 1 hour, in an oxidizing atmosphere containing 3% of oxygenand 97% of nitrogen, for producing the oxygen precipitation nuclei. Thethickness t1 of the silicon oxide film 3 formed by the low-temperatureannealing was measured to be 2 nm by ellipsometry. The resultant siliconepitaxial wafer 60 was subjected to the SC-1 cleaning, and was measuredto have the thickness t2 of the silicon oxide film of 1 nm. The waferwas then subjected to the middle-temperature annealing at 800° C. for 4hours, and further at 1,000° C. for 16 hours, so as to grow the oxygenprecipitation nuclei up to the oxygen precipitate. The density of theoxygen precipitate was evaluated as 2×10¹⁰ cm⁻³ by the infraredscattering tomography.

For comparison, after the silicon epitaxial wafer 50 was obtained underthe same conditions as described in the above, the thickness of thesilicon oxide film measured without being preceded by thelow-temperature annealing at 650° C. for 1 hour was measured to be 1 nmby ellipsometry. The thickness of the silicon oxide film obtained aftersubjecting the silicon epitaxial wafer further to the SC-1 cleaning wasmeasured to be 0.9 nm. The density of the oxygen precipitate obtainedafter subjecting the wafer to the middle-temperature annealing at 800°C. for 4 hours, and further at 1,000° C. for 16 hours, was evaluated as5×10⁶ cm⁻³ by the infrared scattering tomography.

1. A method of manufacturing a silicon epitaxial wafer comprising: avapor phase growth step allowing a silicon epitaxial layer to grow invapor phase on a silicon single crystal substrate manufactured by theCzochralski method, and doped with boron so as to adjust the resistivityto 0.02 Ω·cm or below; a low-temperature annealing step forming,following the vapor phase growth step, oxygen precipitation nuclei inthe silicon single crystal substrate, by carrying out annealing at 450°C. to 750° C., both ends inclusive, in an oxidizing atmosphere for aduration of time allowing formation of a silicon oxide film only to asthick as 2 nm or below on the silicon epitaxial layer as a result of theannealing; and a cleaning step, as the first cleaning step after thelow-temperature annealing step, etching the silicon oxide film formed inthe low-temperature annealing step, using a cleaning solution composedof a mixed solution of ammonia, hydrogen peroxide and water, all ofthese steps being executed in this order, wherein a thickness of thesilicon oxide film after the cleaning step is smaller than the thicknessattained after the annealing step, and is adjusted to 1 nm or above. 2.The method of manufacturing a silicon epitaxial wafer claimed in claim1, wherein particle count on the silicon epitaxial layer after executionof the cleaning step is smaller than the particle count before thecleaning step.
 3. The method of manufacturing a silicon epitaxial waferclaimed in claim 2, wherein SC-1 cleaning solution is used as thecleaning solution.
 4. The method of manufacturing a silicon epitaxialwafer claimed in claim 1, wherein SC-1 cleaning solution is used as thecleaning solution.